How does AHB-AP access cpu core registers

Im learning the debug progress to access memory system in a mcu 

now i know how to read/write a register with a specific address

:write TAR and DRW in the AHB-AP

but how does AHB-AP read/write a core register like pc or sp which have no address

  • Take a look at the debug system registers in the ARMv7-M or ARMv8-M Architecture Reference Manual.

    DCRSR (Debug Core Register Selector Register)

    DCRDR (Debug Core Register Data Register)

     

    Basically you select the register and operation (read/write) and start the transfer by writing to DCRSR. DCRDR holds the data to write or data read.

  • This was the first AP type to be introduced and was first used on Cortex-M based MCUs. The AHB-AP acts like a second memory interface to the system memory and debug registers of the CPU core. In this setup, the AHB-AP and the CPU core share the same 4 GB address space. This means, the debug registers are accessible via the debug interface as well as from the CPU core itself.   official site

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